L26/P-320 Endometriosis and Risk of Autoimmune Diseases: Evidence from a Nationwide Korean Cohort

Abstract Study question Is endometriosis associated with an increased risk of subsequent autoimmune diseases in women, and does this risk vary by autoimmune disease category? Summary answer Women with endometriosis had a significantly higher risk of developing autoimmune diseases, particularly rheumatic and thyroid autoimmune disorders, compared with matched controls. What is known already Endometriosis is a chronic estrogen-dependent inflammatory disorder characterized by immune dysregulation involving both innate and adaptive immunity. Prior studies suggest associations between endometriosis and autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, inflammatory bowel disease, and autoimmune thyroid disorders. However, existing evidence is heterogeneous, often based on cross-sectional or case–control designs, self-reported diagnoses, or Western populations. Large-scale longitudinal data from East Asian populations remain limited, and few studies have comprehensively evaluated multiple autoimmune disease categories within a single analytic framework. Study design, size, duration This nationwide population-based matched cohort study used Korean health insurance data. A total of 33,592 women with endometriosis were propensity score–matched to 33,592 women without endometriosis. Participants were followed longitudinally to assess incident autoimmune diseases using time-varying Cox proportional hazards models. Participants/materials, setting, methods Women diagnosed with endometriosis and age-eligible controls were identified from a nationwide Korean claims database. Propensity score matching balanced demographic, socioeconomic, and clinical covariates. Incident autoimmune diseases were ascertained using diagnostic codes and analyzed overall and by disease category with time-varying Cox regression to reduce immortal time bias. Main results and the role of chance After propensity score matching, baseline characteristics between women with and without endometriosis were well balanced. During follow-up, women with endometriosis experienced a higher incidence of autoimmune diseases than matched controls. In time-varying Cox regression analyses, endometriosis was associated with a 25% increased risk of overall autoimmune disease (adjusted hazard ratio [aHR], 1.25; 95% confidence interval [CI], 1.16–1.36), indicating a statistically robust association unlikely attributable to chance. Disease-specific analyses demonstrated heterogeneity in risk. Endometriosis was significantly associated with autoimmune rheumatic diseases overall, including rheumatoid arthritis, and with Behçet’s disease. Among endocrine autoimmune conditions, an increased risk was observed for Graves’ disease, whereas associations with Hashimoto’s thyroiditis and type 1 diabetes mellitus were not statistically significant. No significant associations were found for systemic lupus erythematosus, ulcerative colitis, or dermatologic and hematologic autoimmune diseases. Confidence intervals excluded the null for the primary outcome and selected disease categories, supporting the robustness of these findings. Null results for other autoimmune diseases may reflect true absence of association or limited statistical power due to low event numbers. Limitations, reasons for caution Disease identification relied on administrative claims without information on severity, stage, or clinical phenotype. Residual confounding by unmeasured factors is possible, and the observational design precludes causal inference. Wider implications of the findings Endometriosis may represent a systemic immune-related condition rather than a purely gynecologic disorder. Clinicians should consider heightened vigilance for autoimmune comorbidities, particularly rheumatic and thyroid diseases, in women with endometriosis. Trial registration number No